A non-aqueous electrolyte lithium ion or lithium secondary battery using a carbon material, an oxide, lithium alloy or lithium metal as an anode is attracting attention as a power source for a cellular phone and a notebook computer because of a higher energy density realized thereby.
A film referred to as a surface film, a protection film, SEI or a membrane (hereinafter referred to as “surface film”) is known to be formed on the cathode surface of the secondary battery. The control of the surface film was known to be indispensable for providing higher technical performances because the surface film largely affects a charge-discharge efficiency, a cycle life and safety.
The reduction of the irreversible capacity is necessary in the carbon material and the oxide material, and problems must be solved with respect to the reduction of the charge-discharge efficiency and to the deterioration of the safety which may be due to formation of dendrite in the lithium metal or the alloy anode.
A variety of techniques have been proposed for solving the above problems. For example, the formation of a film made of lithium fluoride on the surface of the lithium metal or the lithium alloy by using a chemical reaction has been proposed for suppressing the dendrite formation.
JP-A-7(1995)-302617 discloses an anode of which a surface is covered with lithium fluoride prepared by dipping the lithium anode in an electrolyte containing hydrofluoric acid thereby reacting the anode with the hydrofluoric acid.
The hydrofluoric acid is generated by the reaction between LiPF6 and a small amount of water. On the other hand, lithium hydroxide or lithium oxide is formed on the surface of the lithium anode by means of natural oxidation in air. The reaction between them generates the lithium fluoride surface film on the anode surface.
However, the lithium fluoride film formed by the reaction between an electrode interface and a liquid is liable to be contaminated with a side-reaction component in the surface film to hardly provide a uniform film. When the surface film such as the lithium hydroxide and the lithium oxide is not uniformly formed or a part of the lithium is exposed, a problem arises with respect to the safety due to the reaction between the water or the hydrofluoric acid and the lithium in addition to the inability of forming the uniform thin film. When the reaction is insufficient, an unnecessary ingredient may remain to exert ill-effects such as reduction of ionic conductivity.
Further, in the method of forming the fluoride layer by utilizing the chemical reaction on the interface, the choice range of the usable fluoride and electrolyte is restricted and the stable surface film can be hardly formed with a good yield.
JP-A-8(1996)-250108 discloses a surface film made of lithium fluoride on an anode surface generated by the reaction between a mixed gas including argon and hydrogen fluoride and aluminum-lithium alloy.
However, when the surface film is present on the lithium metal surface in advance, especially when a plurality of compounds exist, the reaction is likely to be non-uniform to hardly form the lithium fluoride film uniformly. Accordingly, the lithium secondary battery with the sufficient cycle performance is obtained with difficulty.
JP-A-11(1999)-288706 discloses a technique in which a surface film structure having a sodium chloride crystalline structure component, as a main component, is produced on the surface of a uniform crystalline structure, that is, a lithium sheet having a (100) crystalline plane preferentially oriented. In this manner, a uniform depositing and dissolving reaction or the charge and discharge of the battery can be performed to suppress the dendrite deposition of the lithium metal to improve the cycle life of the battery.
It is described that the material used for the surface film is preferably a halide of lithium and that a solid solution consists of at least one compound selected from LiCl, LiBr and LiI, and LiF is preferably used.
Specifically, in order to produce the solid solution film consisting of at least one compound selected from LiCl, LiBr and LiI, and LiF, the anode for a non-aqueous electrolyte battery is fabricated by dipping the lithium sheet having the preferentially oriented (100) crystalline plane prepared by pressing (rolling) into an electrolyte containing at least one of a chlorine molecule or chlorine ion, a bromine molecule or bromine ion and an iodine molecule or iodine ion, and a fluorine molecule or fluorine ion.
In this technique, the rolled lithium metal sheet is likely to be exposed to atmospheric air. Accordingly, a film originating from moisture is easily formed on the surface to non-uniformize the existence of active points, thereby hardly fabricating the intended stable surface film so that the effect of suppressing the dendrite formation cannot be necessarily and sufficiently obtained.
Techniques for improving the capacity and the charge-discharge efficiency have been reported when a carbon material such as graphite and amorphous carbon which can occlude and release lithium ion is used as the anode material.
JP-A-5(1993)-275077 proposes an anode made of a carbon material the surface of which is coated with a thin film made of a lithium ion conductive solid electrolyte. Thereby, the decomposition of the solvent due to the use of the carbon material is suppressed to provide, especially, a lithium ion secondary battery using propylene carbonate.
However, cracks generated in the solid electrolyte by the stress change during the insertion and desorption of the lithium ion invites the performance deterioration. The non-uniformity such as crystalline deficiency of the solid electrolyte can not provide the uniform reaction on the anode surface, thereby shortening the cycle life.
JP-A-2000-3724 discloses a secondary battery including an anode made of a graphite-containing material, and an electrolyte containing a cyclic carbonate and a linear carbonate, as main components, and further 1,3-propanesultone and/or 1,4-butanesultone I a range from 0.1% weight to 4% in weight.
The 1,3-propanesultone and the 1,4-butanesultone are supposed to contribute to formation of a passivation film on the carbon material surface to cover the active and highly crystallized carbon material such as natural graphite and artificial graphite with the passivation film, thereby suppressing the electrolyte decomposition without impairing the normal reaction of the battery.
Naoi et. el., reported the effect of a complex between a lanthanoid transition metal such as europium and imide anion, to the lithium metal anode in the academic conferences such as the Autumn Conference 2000 of the Electrochemical Society of Japan (September, 2000; Chiba Institute of Technology; Lecture No.:2A24) and the 41st Battery Symposium in Japan (November, 2000; Nagoya Congress Center; Lecture No.:1E03).
The surface film herein made of an Eu[N(C2F5O2)2]3 complex is formed on Li metal dipped in an electrolyte which is prepared by dissolving LiN(C2F5O2)2 acting as a lithium salt into a mixed solvent of propylene carbonate or ethylene carbonate and 1,2-dimethoxyethane and further by adding Eu(CF3SO3)3 as an adding agent thereto.
However, the above prior arts have the following common problems.
While the surface film formed on the anode surface is deeply concerned with the charge-discharge efficiency, the cycle life and the safety affected by its property, no method does not yet exist which can control the film property for a longer period of time.
When, for example, the surface film made of lithium halide or glassy oxide is formed on a layer made of lithium or its alloy, the effect of suppressing the dendrite formation can be obtained at a specific degree at the initial stage of use, however, after the repeated use, the surface film is deteriorated to lower its performance as a protection film.
The reason thereof seems that because the layer made of the lithium or its alloy changes its volume by the occlusion and the release of the lithium while the film thereon made of the lithium halide scarcely changes its volume, the internal stress is generated in the layers and on its interface.
The generation of the internal stress is supposed to damage a part of the surface film, especially, made of the lithium halide, thereby reducing the function of suppressing the dendrite formation.
In connection with the carbon material such as the graphite, an electric charge produced by the decomposition of the solvent molecule or the anion appears as an irreversible capacity component to invite the reduction of the initial charge-discharge efficiency. The composition, the crystalline state and the stability of the film generated thereby largely influence the subsequent efficiency and cycle life.
In the method of forming the organic surface layer on the lithium metal surface investigated by Naoi et al., the effect of improving the cycle life is obtained to a certain extent compared with the other prior art, however, it is not yet sufficient.
Although the researches have been advancing for intending the improvement of the charge-discharge efficiency and the cycle life by forming the film on the anode for the secondary battery, the sufficient performance has not yet been obtained, and the largest problem is how the film of providing the stability and the sufficient charge-discharge efficiency is formed.